In general, audio signal encoding methods such as HE-AAC (High Efficiency MPEG (Moving Picture Experts Group) 4 AAC (Advanced Audio Coding)) (international standard ISO/IEC 14496-3) are known. With such an encoding method, a high frequency feature encoding technology such as SBR (Spectral Band Replication) is used (for example, refer to PTL 1).
According to SBR, when encoding audio signals, SBR information is output for generating high frequency components of the audio signal (hereafter, referred to as high frequency signal) together with low frequency components of the encoded audio signal (hereafter, low frequency signal). At the decoding device, while decoding the encoded low frequency signal, the high frequency signal is generated by using the low frequency signal obtained by the decoding and the SBR information, and so the audio signal made up of the low frequency signal and the high frequency signal is obtained.
This kind of SBR information includes envelope information mainly representing an envelope form for the high frequency components, and noise envelope information representing for obtaining a noise signal added during the generation of the high frequency components at the decoding device.
Here, the noise envelope information includes information representing a boundary position for dividing each SBR frame of the noise signal included in the high frequency components into two zones (hereafter, referred to as the noise boundary position), and information representing gain of noise signals in each zone. Therefore, at the decoding device, a gain adjustment is performed on each zone divided by the noise boundary position on a predetermined noise signal on the basis of the noise envelope information to establish a final noise signal. Further, with SBR, it is also possible to set the gain on the entire SBR frame without dividing the SBR frame of the noise signal into two zones.
When decoding the audio signal, the decoding device generates the high frequency components by combining a pseudo high frequency signal obtained from the low frequency signal and the envelope information, and the noise signal obtained from the noise envelope information, and generates the audio signal from the obtained high frequency components and the low frequency signal.
Also, with SBR, an encoding using sine wave synthesis is performed on an audio signal with a high tone characteristic. That is to say, when generating the high frequency components at the decoding side, a sine wave signal of a particular frequency is added to the pseudo high frequency signal in addition to the noise signal. In this case, the signal obtained from combining the pseudo high frequency signal, the noise signal, and the sine wave signal is set to the high frequency signal obtained as a prediction.
When using a sine wave signal to predict the high frequency components, a sine wave information representing the existence/non-existence of the sine wave signal in the SBR frame is included in the SBR information. Specifically, the combination start position of the sine wave signal used during decoding is either the start position of the SBR frame or the noise boundary position, and the sine wave information is made up of binary information representing the existence/non-existence of a sine wave signal combination in each zone of the SBR frame divided by the noise boundary position.
In this way, the noise signal and the sine wave signal added to the pseudo high frequency signal are components that are difficult to reproduce from the envelope information within the high frequency components of the source audio signal. Therefore, by combining the noise signal and the sine wave signal at a suitable position in the pseudo high frequency signal, it is possible to predict the high frequency components with higher accuracy, and it is possible to reproduce audio at a higher audio quality by performing band pass expansion using the high frequency components obtained by prediction.